1. Field of the Invention
This invention relates to the field of flexible tubes that may be used as air ducts, coolant pipes, fuel tubes, filler necks and the like, such as are used in the automotive industry.
2. Description of the Related Art
Flexible tubes are used, for example, as air ducts in automobiles where the air duct carries air to an engine. The air may pass through a turbocharger, in which case the air flowing through the duct is heated to temperatures of generally 140–160C, but possibly up to 180–200C.
These air ducts are often made by various blow molding techniques. Sequential extrusion blow molding technology is used to produce one-piece air ducts that combine several hard and soft segments.
The internal surfaces of the air duct exposed to the air pressure are under stress. Further, such exposed surfaces create an axial force that causes a longitudinal tension on the air duct because of end-cap effects. As the bellows of the air duct are more flexible than the tube, the bellows tends to elongate more in the longitudinal direction, which is the so-called “pneumatic piston effect”. Expansion of the bellows in the radial direction, called “ballooning”, is relatively low in absolute value compared to the longitudinal elongation of the bellows.
Tensile and shear forces also create constant stress on the air duct which leads to creep of material of the air duct, causing the air duct to elongate over time. This elongation changes the bending stiffness of the air duct, which is why the design of a particular air duct bellows is a case-by-case compromise of various design criteria.
An example of a flexible tube that may be used an air duct is shown FIGS. 1A–1C, where there is shown a flexible tube 11 that includes a bellows 13 having a plurality of convolutes 15 formed in tube 11. Convolutes 15 are, essentially, raised circumferential ridges formed in the surface of tube 11.
When fluids, such as air or liquids, move under pressure through a tube, the pressure in contact with the projected axial surface of tube 11 caused by the end-cap effects creates a force which pulls and deforms bellows 13. This force generates material stress which leads to material creep over time, and which can make tube 11 fall out of its design tolerance. Tube 11 may then move undesirably and contact other parts which may damage tube 11 or the parts it contacts. When used in an engine, the duct may contact hot surfaces or sharp angles and be damaged.
The bellows 15 of flexible tube 11 provides the same bending stiffness along the Y and Z axes. There is no additional design feature that serves to reduce or control the longitudinal deformation, that is, the deformation along the X axis, when flexible tube 11 is subject to elevated temperature and pressure due to the compressed air circulating therein.
One way to overcome the problem of deformation of tube 11 along the X axis is to add two parallel longitudinal ribs to bellows 13 of flexible tube 11. Turning now to FIGS. 2A–2B, there is shown a flexible tube 21 that includes a bellows 23 having a plurality of convolutes 25 formed in tube 21. A pair of opposing longitudinal ribs 27 is formed along bellows 23 so as to connect convolutes 25 with one another. The width and height of ribs 27 may be varied. If rib 27 is wider or higher than convolutes 25, flexible tube 21 will have a greater tensile strength along the X axis but tube 21 will have a weaker burst resistance because rib 27 will tend to unfold or open when subject to high pressure.
Flexible tube 21 provides bending capability along the Z axis, but reduced bending along the Y axis and reduced elongation along the X axis. The design of FIGS. 2A–2B suffers from the problem that it is difficult to design tube 21 to get the right compromise of pressure resistance, stiffness and bending properties.
Another way to reduce the longitudinal deformation of a flexible tube is shown in FIGS. 3A–3C where there is shown a flexible tube 31 that includes a bellows 33 having a plurality of convolutes 35 formed in tube 31. Opposing sides of convolutes 35 have a flattened portion 37 where the outer surface of convolutes 35 is equal to the outer surface of tube 31. This type of flexible tube is the subject of pending patent applications WO 99/22171 and EP 0 863 351 A2.
Flexible tube 31 provides bending capability along the Z axis, but reduced bending along the Y axis and reduced elongation along the X axis. Further, bellows 33 allows tube 31 to be decoupled, that is, it allows tube 31 to move independently of other parts of an engine. Decoupling prevents or greatly reduces the shear stress on the end portions of tube 31.
However, tube 31 suffers from the problems that it is difficult to adjust in terms of pressure resistance, bending stiffness and that the bending stiffness in the Z axis is too high due to limitations in the design and the method of manufacturing such flexible tubes 31.
Flexible tubes such as those discussed above are made by a blow molding process wherein an extruded parison, or extruded tube, of a polymer material is placed in a tool cavity. Turning to FIG. 4A, there is shown a tool 41 having a cavity 43 for making a flexible tube 45. A parison 47 is placed in cavity 43, said parison 47 having a varying thickness so that parison 47 is thicker in the area nearer to the part of the cavity 43 that forms convolutes 48 of tube 45.
Air is blown into cavity 43 and as shown in FIG. 4B, once the blow molding process has been completed, tube 45 is in contact with tool 41, and, due to the internal pressure applied during the transformation, the entire surface of cavity 43 is covered with polymer. As a result of parison 47 being thicker in certain portions so as to accommodate convolutes 48, the wall thickness distribution of tube 45 is not uniform. In particular, a flattened portion 49 of convolute 48 is thicker than any other portion of flexible tube 45.
In general, profiles and shapes close to the initial parison tend to be somewhat thicker than remote ones. The ratio between the location of minimum material expansion, in this case the flattened portion 49 to the location of the maximum material expansion, the thickness of the outer edge of convolute 48, is called the blow ratio and depends on the material used in the blow molding process.
This variation in the wall thickness of flexible tube 31 has the following negative effects:                There is a decrease in the bending capability of tube 45 along the Z axis (shown for example in FIGS. 3A–3C) because the relatively thicker walls of flattened portions 49 are stiffer.        The extra material present in flattened portions 49 is not needed and does not reduce stress on the most exposed area of tube 45, namely the external surface of convolutes 48.        
What is needed, therefore, is, a flexible tube having a bellows design that overcomes the problems associated with the bellows of the prior art.